.
There is one case where they could both have not seen the other. From the video, it appears the plane is descending and the helo is ascending. If they were both flying in close to the same direction with the plane overtaking the helo they might not have seen each other.
The view of the helo from the plane would have been blocked by the nose of the aircraft. The view of the plane from the helo would have been blocked by the helo's roof and engine.
Just heard a report that the helo was asked by the tower if they had the plane in view so there must have been some sort of warning issued.
I know just how fast things can go to hell on the bridge of a ship. It goes even faster in an aircraft.
SpyNavy
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)